For many, many years, humans have sought ways to correct visual problems. The ancient Chinese slept with small bags of mercury on their eyes, flattening their corneas and improving their shortsightedness. Unfortunately, the effects only worked for a few minutes after waking. Spectacles are thought to have been first introduced by the Arabs in the 11th Century and were introduced into Europe about 200 years later. This century has seen the development of contact lenses, initially the hard variety and later soft and disposable soft lenses.
Although these optical aids allow patients to see well while wearing them, they do not offer a permanent cure for the visual disorder or problem. Also, in many situations, they are inappropriate, for example, when swimming or wearing contacts in the laboratory. Another problem is that in some instances dangerous situations can arise when they become dislodged. This can occur while they are being used by firefighters and police officers.
Roughly two decades ago, surgical techniques were introduced in an effort to permanently correct shortsightedness and astigmatism. The radial keratotomy procedure used a diamond blade to make incisions into the cornea, the front surface or "window of the eye". Although this technique worked well, there have been problems with long term stability of vision and weakening of the cornea as a result of the cuts often having to be made up to 95% of the corneal thickness.
More recently, these older techniques have been replaced with laser treatment techniques which have replaced the surgeon's blade with a computer controlled laser that gently re-sculptures the shape of the cornea without cutting or weakening the eye. These laser techniques are typically carried out with a photoablation process using an excimer laser.
Excimer lasers were chiefly developed for the manufacture of computer microchips, where they were used to etch the circuits. However, the laser's extreme accuracy resulted in it being well suited as an eye laser. That is, many eye lasers are extremely accurate and remove only 0.25 microns (1/4000.sup.th millimeter) of tissue per pulse. During the re-sculpturing, the excimer laser gently "evaporates" or vaporizes tissue; there is no burning or cutting involved. In most cases, the laser treatment takes only 20 to 45 seconds, depending on how severe the refractive error is.
In the normal eye, light rays entering the eye are accurately focused on the retina and a clear image is formed. Most of the bending or focusing of the light rays occurs at the cornea, with the natural lens inside the eye being responsible for fine adjustments. If light is not focused on the retina, then the eye is said to have a refractive error. Common refractive errors include: myopia or shortsightedness, hyperopia or farsightedness, and astigmatism. The excimer laser has been used to accurately re-sculpture the cornea in myopia, hyperopia and astigmatism corrections in an effort to make the curve of the cornea focus light rays normally on the retina.
Myopia, or shortsightedness, is a condition whereby light rays come to a focus in front of, rather than on, the retina at the back of the eye. This results in blurry vision, especially when looking at objects far away. Myopia results from the length of the eye being too long or the cornea being too steeply curved.
In hyperopia, or farsightedness, light rays are focused behind the retina. This results in blurry vision especially when looking at objects that are close. Hyperopia results from the length of the eye being too short or the cornea being too flat.
In astigmatism, the cornea, or window of the eye, has an irregular curvature being shaped more like a rugby ball, rather than a soccer ball. Light rays are focused at different points. A person often has some degree of astigmatism and myopia or hyperopia at the same time.
In myopia laser correction procedures, the cornea is flattened to better focus light rays normally on the retina, whereas in hyperopia, the cornea is made more curved. With astigmatism, the surface of the cornea is re-sculptured to a regular curvature.
Under one method of treatment, known as photorefractive keratectomy (PRK), the, laser beam is applied directly to the surface of the cornea, after the thin surface layer of epithelium cells has been removed (e.g., through solvent with wiping, preliminary laser treatment, or minor abrasion). After the direct laser re-sculpturing of the cornea, a bare area of the cornea is left which takes a few days to heal (e.g., 2 to 6 days) and can be uncomfortable during this period. The healing process can sometimes lead to regression (some refractive error returns) or to scarring (which may blur the vision), especially in patients with large refractive errors. Although still used for low degrees of myopia and hyperopia, PRK is generally being replaced by the LASIK method for these same disorders, in which the laser treatment is applied under a protective corneal flap. Under the "Laser in situ Keratomileusis" (LASIK) treatment, a thin protective corneal flap is raised, rather like a trapdoor. The front surface of the exposed cornea is treated by the excimer laser. The net result being that the cornea is altered in a manner directed at allowing light rays to be focused normally on the retina. At the end of the procedure, the protective flap is simply replaced. The LASIK technique leaves the original surface of the cornea virtually intact, hence, there is no bare area to cause pain. In addition, the mild healing process results in minimal regression and avoids scarring problems.
Presbyopia is a problem that is due to an aging process occurring in the natural lens of the eye, and thus is not linked to the cornea being incorrectly shaped as in myopia, hyperopia and astigmatism. As a person ages, the lens expands, becomes harder and less pliable and, because of these factors, is not as capable of changing its shape to focus. In a typical situation, once a person reaches about 40 years of age, the loss of elasticity and the expansion in the natural lens of the eye results in that person experiencing problems with focusing close, for example, during reading. Most people, as they age, suffer from a presbyopia problem. The usual way to correct this problem is to use bifocal lenses. However, some people dislike wearing glasses, particularly bifocals, for many reasons. For example, bifocal lenses present lines where the two portions of the lens are joined together and thus can be unsightly unless more expensive "no line" bifocals are relied upon. Furthermore, people must become accustomed to reading through the one relatively smaller portion of the bifocals.
Because of the underlying differences in the causes for presbyopia and the group of myopia, hyperopia and astigmatism, many ophthalmologists have concluded that there is no cure for presbyopia and that the only solution is to wear reading glasses to compensate for the loss of ability to focus on close objects.
Chapter 4 of the book Surgery for Hyperopia and Presbyopia of Neal A. Sher, M.D., F.A.C.S., 1997 (which book is incorporated by reference in its entirety), describes a corrective presbyopia surgical treatment known as of "Anterior Ciliary Sclerotomy" involving the placement of radial incisions over the ciliary body in an effort to increase the scleral diameter to provide an increased area for ciliary muscle action. This technique is based on the belief that it is not elastic loss in the lens, but a loss in range of action due to the lens' continued growth with respect to a non-growing sclera. The potential complications for this type of a treatment, such as infection, hemorrhage (from cutting to deep), ocular hypotension, myopic shift, and compromise of the limbal conjunctival barrier, makes this technique one that is unlikely to gain wide acceptance.
U.S. Pat. No. 5,314,422 to Nizzola represents one effort to correct presbyopia and involves remodeling, in a PRK process, the front or external surface of the cornea by applying a laser beam through two manipulated plates which together form a sickle shaped aperture. The beam passing through the aperture forms a corresponding sickle shaped recess in an area situated in proximity to the lower part of the pupil rim. The remodeled area thus constitutes a zone of the cornea which functions differently than the rest of the cornea. Thus, this technique simulates a bifocal glasses arrangement and therefore presents the problem of having to shift ones focus from one area to the other depending on the desired viewing object. Also, the technique described in the Nizzola patent is a PRK procedure which removes portions of the outer epithelium layer of the eye and exposed surface of the cornea therebelow and, as a result, complicates and prolongs the healing process which healing process can sometimes lead to scarring and is often uncomfortable to the patient.
Chapter 20 of the aforementioned Surgery for Hyperopia and Presbyopia describes a small diameter intracorneal inlay lens technique used in an effort to correct presbyopia. Under this technique, an incision is made in the eye and a small spatula is utilized to dissect a pocket to the center of the cornea. A small (1.8-2.2 mm diameter) intracorneal hydrogel inlay lens is then placed on the spatula and centered over the patient's pupil. This technique is described as providing a multifocal cornea arrangement. However, this technique is relatively invasive which brings with it the possibility of scarring and infection, and the introduction of a foreign body into the eye is sometimes found unacceptable by some patients.
Chapter 7 of the aforementioned Surgery for Hyperopia and Presbyopia also features a PRK type presbyopia treatment discussion which is directed at creating a defined bifocal or multifocal surface of the human cornea relying on the pseudo-accommodation ability of the patient to be effective. In the PRK treatment described in Chapter 7, a 193 mm excimer laser (MEL 60 Aesculap Meditec, Heroldsbery, Germany) is used. The techniques described include a straight PRK presbyopia treatment, a combination myopic/presbyopic PRK treatment and a combination hyperopic/presbyopic PRK treatment which include rotating and stationary masks designed to form the desired bifocal or multifocal cornea surface topography on the exterior of the cornea with heavy emphasis on a sectorial corneal profiling or on a semilunar cornea steepening profiling to achieve in a defined part of the cornea a presbyopic optical correction. Thus, in some ways this technique is similar to that of the Nizzola method described in U.S. Pat. No. 5,314,422 for presbyopia treatment and thus shares common problems with the Nizzola technique. Additional complications include monocular diplopia and the loss of visual acuity which resulted in some of the clinical tests reported.
Yet another, example of a PRK presbyopia treatment process can be seen in U.S. Pat. No. 5,395,356 which describes a PRK reprofiling of the cornea to create at least one "add" region having a different focal point in an effort to assist the eye in accommodating close-viewing conditions. The "add" region is described as preferably being located near the center of the optical zone and is formed by ablating a profile in Bowman's membrane or Bowman's membrane and adjacent upper portions of the stroma following removal of the outer epithelium layer in a preliminary laser application. As described above, the PRK treatment process, because it leaves exposed ablated areas in the cornea, has associated with it an uncomfortable healing process and the potential for scarring, hazing and infections.
Section IV of the Surgery for Hyperopia and Presbyopia book noted above provides a discussion of an automated lamellar keratotomy (ALK) for hyperopia and laser in situ keratomileusis (LASIK) for correcting hyperopia (Chapters 12 and 13) each of which involves the formation of a corneal flap. As described in Chapter 12, through the work of the present inventor, following upon the earlier work of Dr. Jose Barraquer, nomograms for the correction of hyperopia have been developed for causing a controlled degree of ectasia in the eye to produce a hyperopia correcting steepening of the cornea. The controlled degree of ectasia is based on precise lamellar flap formation which, as described in Chap. 12, preferably involves controlled flap formation through the use of an automated microkeratome such as that described in U.S. Pat. No. 5,133,726 to the present inventor and Sergio Lenchig, which patent is incorporated herein by reference.
The above noted Chapter 13 describes a hyperopia correction procedure, which involves ablating with an excimer laser cornea tissue underlying a displaced lamellar corneal flap formed with a microkeratome like that described above. Following flap formation, a hyperopia correcting mask is attached to an eye fixation suction ring through use of a Meditec handpiece and mask support system. The shape of the mask is dependent on the hyperopia refractive error being corrected, and the laser system directs a sweeping laser beam past the mask which rotates through 360.degree., with adjustable speed (e.g., an angular increment following each laser beam sweep across the mask). Reference is also made in this Chapter to the earlier work of others in the use of LASIK in the treatment of myopia. In this regard, reference is also made to U.S. Pat. No. 4,903,695 to Muller and L'Esperance describing the treatment of myopia, hyperopia and astigmatism through the use of a laser applied to a freshly cut part of the cornea after severing of the lenticle.
Some efforts have also been made to avoid the need for presbyopia correction glasses by programming a laser's computer to leave one eye slightly myopic after treatment, with the other programmed for distance. This is sometimes referred to a monovision, and is often done with patients being subject to a conventional LASIK treatment. However, while helping to avoid the requirement of reading glasses in some patients, a monovision treatment requires some deviation from the desired approximation of normal vision and thus represents a determination that the monoscopic state is not as undesirable as having to use reading glasses. While a minor degree of a monoscopic vision procedure may be useful in supplementing a presbyopia treatment, sole reliance on monovision for correcting presbyopia, however, is undesirable due to the resultant wide variance from normal vision parameters.
The aforementioned U.S. Pat. No. 5,533,997 to the present inventor describes a presbyopic treatment method and system that includes, in one embodiment, a system and treatment method preferably involving flap formation and the controlled formation of an annular ablation in a centralized region of the newly exposed corneal stroma so as to produce an unablated central protrusion of the stroma which transforms the exterior surface of the replaced flap into a multifocal surface that is effective in providing both good near and far sight and is thus effective in avoiding the difficulties imposed by the onset of presbyopia.
As described in U.S. Pat. No. 5,533,997, a preferred treatment technique is one wherein the ablation zone leaves untouched a central corneal area of preferably 1-3 mm and provides a presbyopic corrective ablation ring which has its major depth region also in a relatively central region of the cornea (e.g., a 3.5 mm outer periphery for the presbyopic correcting depth with or without outward additional smoothing). As described in the above referenced application of the inventor, there has been noticed by the inventor that the occlusion of about a 3 mm central area of the cornea does not affect far vision, which led to the realization by the inventor that this is an ideal site for the near vision correction by means of reshaping this zone with a multifocal shape and leaving the most peripheric area of the cornea for intermediate and far vision. In an effort to even further improve upon this earlier work in presbyopic correction in categories such as facilitating laser parameter determination and setup time from patient to patient, helping to avoid mistakes in the setup, and application of an ablation profile well suited for a patient to be treated, hastened healing time, minimizing the degree of postoperative regression, avoiding undesirable reflection or glaring, and generally providing a good near and far vision relationship in the treated eye(s), further study and clinical testing has been carried out with the result being a presbyopic treatment system and technique as described below.